Agriculture is an industry with slim margins and farmers are always looking for ways to cut costs. That’s why in many parts of the U.S., drones are now doing the job of tractors, planters, and planes.

HALIFAX — As a drone glided across farmland on a December day in Truro, N.S., its robotic voice called out, "enable spraying." Two nozzles on the bottom of the drone opened and released a vortex of ...